Last time I went to WDW I went with family had who booked a room at CSR through a conference. When I purchased my hopper pass separately, I got sent a separate card. I even added an individual water park pass at the disney resort and got another card (3 all together by this time). I know my cousins also had individual room keys and park hopper passes. Based on my experience, I don't think you can add them to the card, and if I remember correctly, I even asked. But I could be mistaken, or maybe things have changed.
1992--(First Visit!) Off Site
2002-2006--Disneyland Annual Passholder
2008--*Planned Sept* GF/BCV/VWL
Just joined the DVC June 2008--AKV!